Rate Constants of Atmospheric Chemical Reactions

OH radicals react with almost aU trace molecules in the atmospheric excluding CO2, N2O, CFC, etc., and drives atmospheric photochemical reaction system while the atmospheric lifetime of most of chemical species are determined by the reaction rate with OH, so that OH is the most important reactive species in the troposphere. In this sense, the rate constants of atmospheric molecules and OH are of unequivocal importance. In the stratosphere, inorganic reactions of OH in the HOx cycle and in the cross chain reactions with NOx and ClOxCycIes are also important. The OH reaction rate constants and their temperature dependence with almost all molecules of atmospheric interest have been measured in laboratory. 

Square brackets around a molecular species indicate atmospheric concentration. The rate constants k times the reactant concentration product refers to the rates of the chemical reactions of the indicated number. The photolytic flux term /l4 refers to the photodissociation rate of N02 in Reaction R14, its value is proportional to solar intensity.]. RO2 stands for an organic peroxyl radical (R is an organic group) that is capable of oxidizing NO to NO2. Hydrocarbons oxidize to form a very large number of different RO2 species the simplest of the family is methylperoxyl radical involved in R5, R6 and R8. 

In principle, it is now possible to construct a complete network of interconnecting chemical reactions for a planetary atmosphere, a hot molecular core or the tail of a comet. Once the important reactions have been identified the rate constants can be looked up on the database and a kinetic model of the atmosphere or ISM molecular cloud can be constructed. Or can it Most of the time the important reactions are hard to identify and if you are sure you have the right mechanisms then the rate constants will certainly not be known and sensible approximations will have to be made. However, estimates of ISM chemistry have been made with some success, as we shall see below. 

This section covers some of the more important chemical reactions that occur in the polluted atmosphere and attempts to show how these reactions result in photochemical-oxidant formation. For a more thorough understanding of the chemistry involved, the reader should consult recent reviewsand computer modeling studies by Demeijian, Kerr, and Calvert and by Calvert and MoQuigg. Unless otherwise noted, the mechanisms and rate constants of these modeling studies are used in this discussion. 

Chemical/Physical. Atkinson et al. (2000) studied the kinetic and products of the gas-phase reaction of 2-heptanone with OH radicals in purified air at 25 °C and 740 mmHg. A relative rate constant of 1.17 x 10 " cmVmolecule Sec was calculated for this reaction. Reaction products identified by GO, FTIR, and atmospheric pressure ionization tandem mass spectroscopy were (with respective molar yields) formaldehyde, 0.38 acetaldehyde, L0.05 propanal, X0.05 butanal, 0.07 pentanal, 0.09 and molecular weight 175 organic nitrates. 

Photolytic. The following rate constants were reported for the reaction of 1-pentene and OH radicals in the atmosphere 1.8 x 10cmVmolecule-sec at 300 K (Hendry and Kenley, 1979) 3.14 X 10 " cmVmolecule-sec (Atkinson, 1990). Atkinson (1990) also reported a photooxidation rate constant of 1.10 x 10cmVmolecule-sec for the reaction of 1-pentene and ozone. Chemical/Physical. Complete combustion in air yields carbon dioxide and water. 

Cicerone and Zellner have reviewed the atmospheric chemistry of HCN and discussed its photochemical properties. Penzhorn and Canosa obtained rate constants for the thermal reactions of NO2 with SOj and SO3 using second-derivative u.v. spectroscopy. The relevance of these reactions to aerosol formation in urban atmospheres was briefly discussed. A reassessment of the importance of atmospheric Na constituents has been provided by Kirchhoff and an updated chemical mechanism for the atmospheric photooxidation of toluene has been described by Leone and Seinfeld. ... 

In order to explain the data of Aronowitz et al (12) and previous shock—tube and flame data, Westbrook and Dryer (12) proposed a detailed kinetic mechanism involving 26 chemical species and 84 elementary reactions. Calculations using tnis mechanism were able to accurately reproduce experimental results over a temperature range of 1000—2180 K, for fuel—air equivalence ratios between 0.05 and 3.0 and for pressures between 1 and 5 atmospheres. We have adapted this model to conditions in supercritical water and have used only the first 56 reversible reactions, omitting methyl radical recombinations and subsequent ethane oxidation reactions. These reactions were omitted since reactants in our system are extremely dilute and therefore methyl radical recombination rates, dependent on the methyl radical concentration squared, would be very low. This omission was justified for our model by computing concentrations of all species in the reaction system with the full model and computing all reaction rates. In addition, no ethane was detected in our reaction system and hence its inclusion in the reaction scheme is not warranted. We have made four major modifications to the rate constants for the elementary reactions as reported by Westbrook and Dryer (19) ... 

The half lifetime in air is related to the atmospheric degradation ability of a chemical, measured by the rate constant of its reactions with free radicals (e.g., OH NO3) and ozone O3 or of photochemical reactions [Gramatica and Papa, 2007 Gramatica, Pilutti et al., 2003b]. 

VOCs are removed and transformed in the troposphere by photolysis and chemical reaction with OH radicals, NO, radicals, and O,. In the presence of sunlight, the degradation reactions of the VOCs lead to the conversion of NO to NO2 and the formation of O,. However, different VOCs react at differing rates in the troposphere because of their differing rate constants for photolysis and reaction with OH radicals, NO, radicals, and O,. The rate of ozone production from a given VOC is essentially a function of three factors the species atmospheric concentration, its rate of reaction with OH (and NO, and O,), and the number of ozone molecules produced each time the species is oxidized. VOCs exhibit a range of reactivities with respect to the formation of O,. 

Theory and calculations on the chemical reactions of polyatomic molecules are very active areas of research, " There are several reasons for this. The most modem experimental techniques using lasers and molecular beams are being applied to study the microscopic details of such chemical reactions including how different vibrational modes of polyatomic molecules influence reactivity," and measurements of the lifetimes of reaction complexes. State-selected experiments of this type require detailed quantum reactive scattering theory in their interpretation. Furthermore, there is a need for the accurate calculation of kinetic data such as rate constants of polyatomic reactions that are sometimes difficult to study in the laboratory but are important in areas such as atmospheric, combustion, and interstellar chemistry. 

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